1. Field of the Invention
The invention relates to a special process for the cutting production of threads, in which, at least in a part region of their extension, a more or less sliding influence on or correction of the thread profile is desired. Such a design can be advantageous for specific uses. Such a preferred use of the process is proposed.
2. Description of the Related Art
Threads are in widespread use as constructive elements in general mechanical engineering. Threads are usually made cylindrical, and in addition conical threads are also commonly used, for example for oil-field pipes. A large number of different thread profiles are known and are laid down in standards. The said profile on a workpiece is usually invariable, that is to say the thread profile at the thread start is identical to that at the thread end. Exceptions are conceivable, however, in which it could be advantageous to have a shape of the thread profile formed from thread groove and thread tooth which changes in a flowing manner at least in a part region of the thread, for example in order to make it easier to introduce a screw thread into a nut thread.
However, special geometrical conditions with respect to the thread prevail primarily in threads on curved surfaces, such as occur particularly in the case of screw-in artificial hip-joint sockets. Here, with regard to the outer shape of the shell body, for example hypospherical, hemispherical or hyperspherical, conico-spherical, parabolic, toroidal, elliptic or similar geometries are known. Metal-cutting production processes for the threads of screw sockets of this type result, sometimes necessarily, in distortions of the thread profile which vary in flowing manner and which in most cases are neither intended nor desired. Particularly the use of thread teeth having asymmetric flank angles, there is the phenomenon that, depending on the tilting direction of the resulting thread tooth, the tooth height from the socket equator towards the socket pole increases or decreases in a flowing manner, the result of this then being either much too large or virtually stunted thread teeth at the near-pole thread start. In the first case, extremely large thread teeth led to the fact that very high forces become necessary in order to screw in the socket or the implant cannot be screwed in until there is full bone contact. In the second case, only very feeble primary fixation is to be achieved. In both cases, there is the risk that the implant will work loose, and this would mean a further operation on the patient as a consequence.